Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Continuous Phase Modulation (CPM) is a method for modulation of data commonly used in wireless communications systems. For example, CPM is used in wireless modems of the wireless communications systems. In contrast to other coherent digital phase modulation techniques wherein the carrier phase abruptly resets to zero at the start of every symbol, with CPM the carrier phase is modulated in a continuous manner. For instance, with a coherent digital phase modulation technique, such as Quadrature Phase-Shift Keying (QPSK), the carrier instantaneously jumps from a sine to a cosine, i.e. a 90 degree phase shift, whenever one of the two message bits of the current symbol differs from the two message bits of the previous symbol. This discontinuity requires a relatively large percentage of the power to occur outside of the intended band, e.g., high fractional out-of-band power, leading to poor spectral efficiency. Furthermore, CPM is typically implemented as a constant-envelope waveform, i.e. the transmitted carrier power is constant. CPM is advantageous because the phase continuity yields high spectral efficiency, and the constant envelope yields excellent power efficiency. However, a drawback is the high implementation complexity required for an optimal receiver.
CPM is a non-linear digital modulation method in which the phase of the signal is continuous. As mentioned above, it has excellent spectral characteristics. One of the most notable advantages of CPM is that it has constant envelope waveform, and therefore it is appropriate for use in transmitters using non-linear power amplifiers. For these reasons CPM is widely used in cellular communication systems and in satellite communication systems.
Transmit diversity is radio communication using signals that originate from two or more independent transmitters that have been modulated with identical information-bearing signals. Further, the signals may vary in their transmission characteristics at any given instant.
Transmit diversity may overcome the effects of fading, outages, and circuit failures. When using diversity transmission and reception, the amount of received signal improvement depends on the independence of the fading characteristics of the signal as well as circuit outages and failures.
In many communications systems, adding antennas to a receiver or a transmitter may be expensive or impractical. In such cases, transmit diversity using multiple transmit antennas may be used to provide diversity benefits at the receiver.
Since transmit diversity with N transmit antennas results in N sources of interference to other users, the interference environment will be different from conventional communication systems comprising one transmit antenna. Thus, even if transmit diversity has almost the same performance as receive diversity in noise-limited environments, the performance in interference-limited environments will differ.
Thus, transmit diversity techniques improve link performance without the need to add extra Radio Frequency (RF) equipment at the receiver, e.g. a mobile unit. A transmit diversity scheme specifically designed for channels with time dispersion was introduced in Lindskog and Paulraj, “A Transmit Diversity Scheme for delay Spread Channels”, in Pro. IEEE Int. Conf. Commun. (ICC 2000), June 2000.
Below some properties of CPM and prior art on transmit diversity techniques will be described.
Decomposition of CPM Signals into Pulse Amplitude Modulation (PAM) Waveform
This section relates to a brief review of some background material that is needed in order to understand some embodiments which will be described herein. A detailed exposition of the material in this section is found in “Decomposition of M-ary CPM Signals into PAM Waveforms” (Mengali U., and Morelli, M., IEEE Transaction on Information Theory, vol. 41, no. 5, 1995).
Given a bit sequence {right arrow over (a)}={ak}k=0N, akϵ{0,1}, a complex baseband CPM signal has the form s(t)=exp(jψ(t,{right arrow over (α)})) with
            ψ      ⁡              (                  t          ,                      α            →                          )              =          2      ⁢      h      ⁢                          ⁢      π      ⁢                        ∑                      n            =            0                    N                ⁢                              α            n                    ⁢                      q            ⁡                          (                              t                -                nT                            )                                            ,where h is the modulation index, T is the signaling interval, αk=1−2ak, αkϵ{−1,1} are the information symbols and q(t) is the phase pulse. The phase pulse is related to the frequency pulse f(t) by the relation
      q    ⁡          (      t      )        =            ∫              -        ∞            t        ⁢                  f        ⁡                  (          s          )                    ⁢      d      ⁢                          ⁢              s        .            The frequency pulse f(t) is time limited to the interval (0, LT), where L is a positive integer. When L=1, the CPM signal is called full response CPM. Otherwise it is called partial response CPM.
A CPM signal, s(t), may be decomposed into a superposition of PAM waveforms using the Laurent decomposition (“Decomposition of M-ary CPM Signals into PAM Waveforms” (Mengali U., and Morelli, M., IEEE Transaction on Information Theory, vol. 41, no. 5, 1995)), whenever the modulation index h is not an integer. This decomposition takes the following form.
                                          s            ⁡                          (              t              )                                =                                    ∑                              k                =                0                                            Q                -                1                                      ⁢                                          ∑                n                                                                              ⁢                                                b                                      k                    ,                    n                                                  ⁢                                                      c                    k                                    ⁡                                      (                                          t                      -                      nT                                        )                                                                                      ,                            (                  Equation          ⁢                                          ⁢          1                )            where Q=2L−1, Ck(t) are a set of pulses whose explicit definition is found in Eq (7) of “Decomposition of M-ary CPM Signals into PAM Waveforms” (Mengali U., and Morelli, M., IEEE Transaction on Information Theory, vol. 41, no. 5, 1995), and bn,k are the so-called pseudo-symbols. The pseudo-symbols, bn,k, depend on the information symbols, αkϵ{−1,1}, in a non-linear way. For a given 0≤k≤2L−1, the expansion of k in binary digits can be expressed as
  k  =            ∑              i        =        1                    L        -        1              ⁢                  2                  i          -          1                    ⁢              β                  k          ,          i                    for some binary coefficients βk,iϵ{0,1}. The pseudo-symbols are defined by
                              b                      k            ,            n                          =                              exp            (                          j              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              h                [                                                                            ∑                                              m                        ≤                        n                                                                                                                                  ⁢                                          α                      m                                                        -                                                            ∑                                              i                        =                        0                                                                    L                        -                        1                                                              ⁢                                                                  α                                                  n                          -                          i                                                                    ⁢                                              β                                                  k                          ,                          i                                                                                                                    ]                                      )                    .                                    (                  Equation          ⁢                                          ⁢          2                )            
The Laurent decomposition is not defined if h is an integer, see the paper “Decomposition of M-ary CPM Signals into PAM Waveforms” (Mengali U., and Morelli, M., IEEE Transaction on Information Theory, vol. 41, no. 5, 1995), Section II B.
Full response CPM (i.e. L=1) has a particularly simple Laurent decomposition, since Q=2L−1. In this case, Equation 1 and Equation 2 yield that the CPM signal, s(t), may be expressed as
                              s          ⁡                      (            t            )                          =                              ∑                          n              =              0                        N                    ⁢                                    exp              (                              j                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                h                ⁢                                                      ∑                                          m                      ≤                      n                                                                                                                      ⁢                                      α                    m                                                              )                        ⁢                                                            c                  0                                ⁡                                  (                                      t                    -                    nT                                    )                                            .                                                          (                  Equation          ⁢                                          ⁢          3                )            
Even though the Laurent decomposition of a partial response CPM signal is more complex than its full response counterpart, the expression in Equation 3 is approximately valid for partial response CPM signals in many cases of practical interest. These cases include the Gaussian Minimum Shift Keying (GMSK) modulations used in Global System for Mobile Communications (GSM) and Digital Enhanced Cordless Telecommunication (DECT). In general, Equation 3 provides an accurate representation of a partial response CPM signal whenever the energy, that is, the second norm, in the pulses Ck(t), 1≤k≤2L−1, is much less than the energy in the main pulse C0(t).
Lindskoq-Paulraj (LP) Transmit Diversity
In “A Transmit Diversity Scheme for delay Spread Channels” (Pro. IEEE Int. Conf. Commun. (ICC 2000), June 2000), Lindskog and Paulraj developed a transmit diversity scheme for channels with dispersion. The diversity scheme achieves full second-order receive diversity with one receive antenna and two transmit antennas. A symbol stream d(t) of complex-valued symbols to be transmitted is fed to a space-time encoder. The space-time encoder divides the symbol stream into two symbol streams; d1(t) and d2(t), each symbol stream containing half of the symbols. The transmission frame is also divided into two blocks. The space-time encoder provides input to two transmitters. The space-time encoder transmits, during a first block of the transmission, the time reversed, negated and complex conjugated second symbol stream −d*2(N−t) from a first transmit antenna, and the time reversed, complex conjugated first symbol stream d*1 (N−t) through a second transmit antenna. The encoder transmits, during a second block of the transmission, the first symbol stream d1(t) from the first transmit antenna and the second symbol stream d2(t) from the second transmit antenna. FIG. 1 schematically illustrates the transmit diversity according to Lindskog-Paulray. Furthermore, the paper “A Transmit Diversity Scheme for delay Spread Channels” (Lindskog and Paulraj, in Pro. IEEE Int. Conf. Commun. (ICC 2000), June 2000) discloses the necessary receive processing.